1. Field of the Invention
The present invention relates to earth boring bits used in the retrieval of oil, natural gas, and other earth minerals. In particular, the invention is a new rolling cutter drill bit with a tooth structure having integral secondary cutting elements and a reduced stress tooth geometry.
2. Description of the Related Art
Earth boring bits used for the recovery of petroleum and other minerals from the earth are generally of either the fixed cutter type or rolling cutter type. A fixed cutter drill bit has a plurality of cutting edges that are pushed into and dragged through the earth. This type of bit removes the earth primarily by shearing and scraping.
Rolling cutter drill bits (also known as rock bits) have cutting teeth or cutting inserts arranged upon one or more rotating cutters. Typically, the bit body has three cantilevered bearing spindles with frusto-conically shaped rolling cutters arranged on them. When the bit body is rotated and weight is applied, the teeth on the cutters engage the earth causing the rolling cutters to rotate. As the cutters rotate, the teeth are sequentially pushed into the earth, effecting a drilling action.
There are two broad classifications of rolling cutter drill bitsxe2x80x94insert bits and tooth bits. The distinction between these bit types is in the way in which the cutting teeth are formed on the rolling cutter. Insert bits typically have cylindrical tungsten carbide cutting elements interference fitted into holes drilled into the cutters. These bits typically drill very hard and tough rock formations where the durability of the superhard cutting elements is required for good bit life and performance. The limited protrusion, blunt cutting edge, and narrow width of the cutting inserts, however, limit how fast the bit can drill.
On a tooth type rock bit, the cutting teeth are formed out of the same steel material as the frusto-conical roller body. Tooth type rock bits are generally used in the softer drilling applications where the long, sharp teeth allow deep penetration into the formation. The deep penetration into the earth combined with a wide tooth profile provide high drilling rates of penetration for tooth bits in the softer formations. The relatively slender teeth are designed primarily to drill the softer formations. Encounters with hard, tough formations often cause very rapid wear and/or breakage of the teeth.
In one commonly used process for manufacturing tooth type bits, the teeth and cutter body are machined from a blank of forged steel. Multiple machine milling passes are required to remove all the material from the blank to form the teeth and other surface features.
After machining, portions of the teeth are provided with hard, wear and abrasion resistant coatings to slow the degradation (or dulling) of the teeth as the earth is being drilled. On most tooth type bits, this coating is applied to the teeth with a weld process. After welding the hardmetal coatings on the teeth, portions of the cutters are carburized, and the whole cutter is hardened in a heat treatment process. The bearing surfaces inside the cutter are then finished machined. Finally, the cutters are assembled onto the bit bodies with one of a number of methods well known in the rock bit manufacturing industry.
Numerous U.S. Patents, such as: U.S. Pat. Nos. 3,946,820; 3,412,817; 3,401,926; 3,266,342; 3,223,188; 3,003,370; 2,097,551; 1,885,085; 1,957,532; 2,107,788; 2,244,617 (all herein incorporated by reference), show a variety of designs for tooth bit teeth and/or tooth bit machining process.
Another method for manufacture of tooth type rock bit cutters is rapid solid state densification powdered metallurgy (RSSDPM). The RSSDPM process and products made by it are shown and described in U.S. Pat. Nos. 4,554,130; 4,562,892; 4,592,252; 4,630,692; 5,032,352; 5,653,299 and 5,676,214 all herein incorporated by reference. RSSDPM has been proven a commercially viable means of forming tooth type bit cutters. One advantage to rapid solid state densification is the teeth can be formed with an integral hard coating as part of the manufacturing process. This type of hard coating eliminates the hardfacing welding step required in machined tooth cutters and produces a very smooth surface on the tooth. Integral formation of the hard coating improves the performance of tooth bits and the increased percentage of tungsten carbide available at the tooth surface increases the wear resistance.
The RSSDPM process also allows greater flexibility in tooth bit cutter design. For instance, in U.S. Pat. No. 5,767,214, the RSSDPM process allows a cutter design with channels formed in the gage face and angled teeth that would at best be impractical to make with a machining process. As noted in the prior art, the RSSDPM process produces a tooth type bit cutter with numerous properties that are separate and distinct from all other known process.
The drilling applications for tooth type bits have been limited by the inability of these bits to drill hard rock. In most areas of the earth, the sub-surface rock formations are layered. It is very common for an extremely hard layer of rock to be adjacent to a soft layer of rock in the earth. In the drilling industry, it is desirable to drill through both layers as quickly as possible. Since it is time consuming (and therefore expensive) to change the rock bit each time a new layer of rock is encountered, a bit chosen to drill both types of formations is often running under less than optimum drilling conditions.
In the cases where the majority of the drilling is hard rock, the insert bit will simply drill slower when drilling in the soft layers. However, when the majority of the layers are soft rock, a tooth bit can be destroyed by a relatively thin layer of hard, tough rock. The driller then faces a dilemma. If he uses an insert type bit he can easily drill through the hard formations, but he will sacrifice drilling speed and drive up the cost. If the driller uses a tooth type bit, he will get economical, high drilling speeds but he risks destruction of the bit and the downtime required to replace it.
Although advances in hardfacing materials and methods, and the advent of RSSDPM processing have made tooth type bits more durable in recent years, the problem remains. There is a need in the drilling industry for a tooth type bit that has the broad, long cutting teeth required for optimum drilling performance in soft formations that can also survive short intervals of extremely hard, tough rock drilling.
The present invention is a tooth type drill bit with secondary cutting elements on the tooth surfaces. The secondary cutting elements provide additional, relatively sharp edges that help cut hard rock.
The teeth also have convex radiused buttresses at their bases to reduce the stresses the teeth experience when drilling through extremely hard rock. This allows the tooth bit to survive the high loads encountered when drilling hard, tough rock formations, without breaking the teeth.
The combination of teeth with secondary cutting elements and convex radiused buttresses at their base provide a tooth type bit that has superior resistance to wear and breakage when drilling through hard, tough rock without significantly reducing the drilling rate of penetration when drilling through soft rock.
In its broadest sense, the invention is a tooth type rolling cutter drill bit with a plurality of rolling cutters mounted on bearing spindles. At least one generally wedge-shaped cutting tooth with a top, sides, and a base is formed on one of the rolling cutters. The sides of the tooth comprise a pair of substantially flat flanks and a pair of curved end surfaces. The flanks are joined at one end to form an elongated cutting edge with a given minimum thickness at the top of the tooth. A buttress is formed about the base of the tooth. The flanks and sides of the tooth smoothly blend into the buttress. The buttresses and at least one of the curved end surfaces of the tooth each forms a convex surface.
A secondary cutting element may be formed on a flank of the tooth between the elongated cutting edge and the buttress. The secondary cutting element protrudes from the flank and has a secondary cutting edge with a top end adjacent to, but not touching, the elongated cutting edge of the tooth and a bottom end that is adjacent to the buttress. The secondary cutting edge is aligned generally perpendicular to the elongated cutting edge of the tooth. The maximum protrusion of the secondary cutting element is no greater than the thickness of the elongated cutting edge of the tooth. The bottom end of the secondary cutting element may be wider than its top end. The protrusion of the secondary cutting edge may be greater at its bottom end than its top end. The secondary cutting element is formed of a hard, wear and erosion resistant material, preferably tungsten carbide in an iron alloy matrix.
All, or a portion of the tooth, may be formed with a hard, wear and erosion resistant surface, preferably tungsten carbide in an iron alloy matrix - in an RSSDPM process.